The present invention relates to an improved porcelain fused to metal crown which is useful for an artificial tooth crown in the restoration of a tooth crown.
When the tooth crown is partially or wholly lost owing to various diseases, such as dental caries and paradental disease, traffic accidents or the like, the defective portion is restored with an artificial material for esthetic purposes and for recovering the mastication function and the utterance function. When the defect of the tooth crown is small, it is remedied by an inlay technique, and when the defect thereof is large, it is remedied by prosthetic dentistry, such as post crown, bridge denture and plate denture.
The enamel portion of natural teeth has a knoop hardness of 300 to 350, and among the dental materials, only porcelain has an excellent hardness comparative with that of natural teeth. Furthermore, porcelain has superior chemical stability and low thermal conductivity and is more preferable from an esthetic viewpoint, such as color and transparency, in comparison with the metallic restorations.
Although porcelain has an excellent compressive strength, it is inferior in tensile strength and shearing force. In about 1950, a porcelain fused to metal crown was developed in The United States of America. The porcelain fused to metal crown has both characteristics of excellent strength owing to the metal and the excellent esthetic properties similar to natural teeth and excellent wear resistance owing to the porcelain, by which the brittleness of the porcelain is complemented with the strength of the metal, and therefore, it has been widely used. However, it has still some defects. For instance, the alloys used therefor should have a higher melting point than the baking temperature of porcelain. Moreover, as the metal crown, there have been used the products made of noble metal alloys (e.g. platinum-gold alloy) because of the good compatibility thereof with porcelain, but the metal crown should be subjected to troublesome pretreatments, such as the treatment with hydrofluoric acid and the degassing treatment. They have further defects, such as cracking or breaking of the porcelain layer, which may be owing to the deformation of the crown due to the difference in the thermal expansion coefficient between the metal crown and the porcelain and the mechanically intermeshing pressure.
As the results of the present inventors' extensive studies, it has been found that a porcelain fused to metal crown having a composite structure comprising layers of a bonding agent and ceramics thermally sprayed around the metal crown and a layer of fused porcelain thereon can give an excellent artificial tooth crown excellent mastication properties, and aesthetic requirements and further having superior durability in comparison with the conventional porcelain fused to noble metal crown, even when using cobalt-chromium alloys and nickel-chromium alloys which have hitherto been considered to be difficult in the baking of said alloys onto porcelain. Furthermore, the artificial tooth crown has an excellent bonding strength between the porcelain layer and the metal crown without requiring the degassing treatment and the treatment with hydrofluoric acid even by using noble metal alloys as used in the conventional procelain fused to metal crown.
An object of the present invention is to provide an improved porcelain fused to metal crown having a composite structure comprising layers of a bonding agent and ceramics thermally sprayed around the metal crown and a layer of fused porcelain thereon.
Another object of the present invention is to provide an artificial tooth crown having excellent durability, mastication properties and aesthetic requirements.
A further object of the present invention is to provide an artificial tooth crown using cobalt-chromium alloys or nickel-chromium alloys for the metal crown.
A still further object of the present invention is to provide a method for preparing a porcelain fused to metal crown.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art feom this detailed description.
In the present specification and claims, the term "porcelain fused to metal crown" denotes an artificial tooth crown, including various types of bridge.
The porcelain fused to metal crown has a composite structure comprising layers of a bonding agent and ceramics, and optionally a mixture of a bonding agent and ceramics, which are thermally sprayed around the metal crown, and a layer of fused porcelain formed on the above layers.
With reference to the accompanying drawings, the porcelain fused to metal crown of the present invention is illustrated.